Treatment of eye disorders can pose significant problems. For example, the management of many corneal diseases requires that therapeutic agents, such as drugs or other medications, remain at therapeutic levels in the corneal tissue for extended periods of time. Topical drops remain the most common route of drug administration to the eye, especially the cornea. Certain patients, however, including those patients having fungal and/or amoebic infections of the cornea, as well as corneal transplant recipients, require therapeutic doses of medications to be continuously maintained in the corneal tissues. To achieve such therapeutic doses of a particular medication in corneal tissues, such patients are required to endure lengthy and arduous dosing regimens that often involve topically instilling medications hourly.
The efficacy of topical dosing can be diminished by poor patient compliance. The repeated administration of one or more medications can also lead to toxicity, which can be related to excess dosing of the medications and/or their preservative agents. Injections of free-standing medications into the corneal stroma also have been employed in certain situations. This approach, however, fails to result in sustained presence of medications within the therapeutic window due to rapid diffusion of the medication out of the cornea. The combination of poor compliance and poor penetration of topically-administered medications into the corneal stroma can result in treatment failure, even when the pathogenic organism or other cause of disease is known to be “sensitive” to the therapy or should otherwise respond to the medication being applied to the eye.
Such treatment difficulties are not only observed with corneal diseases. Disorders common to the anterior chamber (AC) can also pose problems. For example, cataract extraction is one of the most commonly performed surgical procedures in the United States. Both the incidence of cataracts and the frequency of their extraction surgeries continue to rise. Anti-inflammatory agents and antimicrobials both are required during the post-operative period and typically must be administered for many weeks. Unfortunately, the poor ocular bioavailability of these drugs often necessitates complicated and arduous dosing regimens that can lead to poor patient compliance, which, in turn, ultimately contributes to poor surgical outcomes and other complications. Current state-of-the art treatment involves sustained delivery systems that are injected into the subconjunctival space at the end of surgery. However, such sustained release drug systems afford limited intraocular bioavailability of delivered medications due to barriers to drug penetration. Further, medications delivered to an extraocular site, even in a sustained manner, will have lower intraocular bioavailability due to barriers that prevent penetration.
Injecting medications directly into the AC of the eye at the end of surgery has been an alternative to topical treatment. Free-standing medications injected into the AC, however, are readily eliminated during aqueous humor clearance. As a result, the retention of medications injected into the AC is short and their bioavailability fails to meet clinical guidelines.
The delivery of medications to the posterior segment of the eye has also been a challenge for ophthalmologists. Currently, the successful management of many retinal diseases often requires multiple intravitreal injections of medication per year. Retinal diseases, such as diabetic retinopathy and age-related macular degeneration, among others, continue to represent major causes of irreversible vision loss in the United States. The treatment of these conditions is complicated by the difficulty of delivering medications to the retina. The direct delivery of drugs to the vitreous chamber has become an important tool in the arsenal of retinal disease management.
Unfortunately, the chronic and progressive nature of many retinal diseases necessitates repeated injections to provide continued treatment. The current regimens, which vary based on the disease, its stage, and the medications used, can include up to six intraocular injections per year. Each intravitreal injection, however, is associated with the risk of developing serious and vision-threatening infections and even traumatic retinal injury.